Battery line variations in a vehicular environment are quite common, and many types of circuitry must correctly operate under these conditions. This problem is no more pronounced than during the starting of a vehicle engine, wherein the starter motor typically draws the most current from the vehicle battery. In this case the load is very high, dropping the battery voltage severely. However, there are many circuits in a modem automobile that are required to operate, even under a high cranking load.
During starting there are several circuits, other than the starter circuit, that need to operate properly, even under the constraint of very low available voltage. For example, certain crank related functions such as the engine control module (ECM) and starter solenoid control must be operable during engine starting. If there is insufficient voltage available, the main module power supply for the ECM will not have sufficient voltage to operate the controller, which will cause the circuit to enter a low (input) voltage reset. Without sufficient voltage the ECM will not be operational, and the vehicle will not start.
One critical function is the control of a Power Hold Relay (PHR) that supplies power to several of these critical circuits. For example, when the PHR coil is energized and contacts closed, the PHR supplies the input power to the ECM main voltage regulator, which in turn supplies operational voltage to the main controller. The controller can also control the PHR output driver to power the PHR once the controller is operational. In operation, before starting, the vehicle controller is in a RESET state. When the ignition switch of the vehicle is first turned to the RUN position, power can be supplied directly to the output driver to the PHR coil through a dedicated line. Then after the voltage to the main supply regulator comes up, the main controller will leave the RESET mode and provide an output signal to keep the PHR output driver turned on.
In practice, during the initial phase of the starter cranking operation, when power is first applied to the starter motor, the available battery voltage can dip down to (or below) three to five volts, until the starter motor starts to turn and develop a back electromotive force (EMF). During this power dip the main supply voltage regulator typically goes into an under voltage RESET condition, turning off the main controller. As the starter motor starts to rotate, the developed motor back EMF reduces the load on the battery and allows battery voltage to return to a level where the main supply voltage regulator can work; releasing the RESET signal to the main controller.
During the initial phase of the engine cranking, during the power dip, it is not actually necessary to keep the main controller operational. All that is necessary is to provide a means to keep the PHR and Starter Motor Control Relay output driver circuits operational, while the rest of the module may be held non-operational in the RESET state. Once cranking continues battery voltage will recover enough to allow the module to leave the RESET state and drive the power and control relays directly.
Typically, vehicle manufacturers want to ensure that any such Power Hold Relay and Starter Motor Control Relay output drivers work, for some specified. period of time where the available battery voltage is below that which is normally required to engage the relays (i.e., function becomes non-operational because of relay limitations, not electronics limitations). Thus, if the supply/battery voltage drops to a level below which the PHR stays engaged, the power to operate the PHR output driver needs to come from the ignition switch RUN position input.
Moreover, complications arise with supply/battery voltage transients, particularly during cold weather engine cranking, wherein the PHR output driver is still required to operate during these transients. Further complications arise in some additional manufacturer requirements such as where: a) the Power Hold Relay and Starter Motor Control Relay output driver circuits need to be self-protecting; since the main controller is not always operational when these outputs are active, wherein output driver protection is not allowed to be a fold back protection scheme, even with automatic retry; b) it is required that the “Power Hold Relay” output driver circuit be capable of turning OFF within a specified period of time after the ignition RUN Position switch opens (i.e. the main controller is not running or not running in the hold ON mode) and/or after the main controller relay hold output signal is removed; and c) the ignition RUN Position switch input also needs to provide a logical input signal to a main controller, and potentially to one or two other secondary controllers in the module. It would be desirable (but not always possible) that the input threshold voltage for turning ON the relay driver output is also the same threshold voltage used to signal the controllers that the switch is open or closed.
Engine control modules with delayed Power Hold Relay functions and circuits are not new. However, as additional requirements are placed on the operation of the PHR function, and in view of the complications that can arise, new and unique solutions are required.
FIG. 1 shows a basic circuit for supply voltage during low battery voltage conditions. This solution typically depends on a large charge storage capacitor to store charge for those times when there is insufficient battery voltage. Such charge storage capacitor is charged up just prior to the cold cranking supply/battery voltage dip. Specifically, a B+ supply from a battery for example (e.g. 12 volts) is connected from the ignition switch RUN position through a forward-biased blocking diode D1. During operation, B+ supplies power to an output, coupled to the PHR output driver, for example. A high power zener diode D2, and an electrolytic storage capacitor C1 are connected in parallel, in a shunt configuration, between the B+ line and the output. Other devices (not shown) can also be included as are known in the art for current limiting, etc.
This circuit supplies voltage during low B+ conditions by storing charge in the electrolytic capacitor C1. Capacitor C1 must be large enough to supply sufficient voltage during starter cranking until starter EMF drives battery voltage recovery. This may be several hundred milliseconds, which requires a large capacity, resulting in the requirement for a physically large capacitor with a high cost. Moreover, the total voltage available with a single capacitor is limited to no more than the B+ supply. In addition, this circuit does not address any of the complications that can arise.
Another solution is to charge a capacitor to a voltage greater than the supply voltage by a voltage step-up or “boost switcher” power supply, which can utilize. smaller capacitors with a higher voltage rating. Typical step-up circuits include a charge pump circuit and a voltage regulating and current limiting circuit, as are known in the art. In this case, power from the ignition RUN Position switch would initially turn the PHR driver circuit ON and once the boost supply is up and operational, the running (not in reset) main controller circuits can be used to keep the driver output ON during the battery supply dip phase of the cold cranking operation.
However, having to use a boost switcher power supply to meet these simple requirements is excessively cost prohibitive. Additionally, if the PHR should drop out (during the initial cranking voltage dip) the switching supply can drop out and could take tens of milliseconds to reestablish its operation after the PHR contacts close again, potentially resulting in a “relay chattering” non-operational mode. In addition, this solution does not meet the requirement of the relay output driver circuit staying ON, even when the contacts of the relay open.
Another possible solution is to use a diode powered “OR” circuit to the input pin of a relay “smart” output driver circuit. This configuration uses one of the diode inputs connected to the ignition RUN Position switch and another input diode connected to a controller hold signal output pin.
Typical “smart” self-protected output driver ICs derive their operational power from the voltage at their input pin. However the required voltage at the input pin (worst case high) of such a “smart” driver, plus one diode drop, is typically above that which may be available during a cold crank supply/battery dip. Therefore, capacitors need to be added, after the RUN Position input diode, to hold the output driver on during a supply/battery dip. However, the capacitors can also be charged by the micro controller output, resulting in too long of a delay time from when the micro controller output goes low to when the relay output driver turns OFF. Therefore, more diodes are needed, which now are required to be low-drop Shottkey diodes with reverse leakage current concerns. Further, resistors are needed to obtain added protection of the “smart” output driver and for discharging the charge storage capacitors to limit maximum time until turn OFF. All of these solutions needlessly add cost to provide the added protections required.
Therefore, it is desirable to provide a simpler, less problematic circuit approach that overcomes most, if not all, of the preceding problems. It would also be beneficial if a technique could be provided such circuit in a low cost configuration.